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Sound Reinforcement - Forums for Live Sound Professionals - Your Displayed Name Must Be Your Real Full Name To Post In The Live Sound Forums => LAB: The Classic Live Audio Board => Topic started by: Lee Richard on August 09, 2015, 03:50:49 am

Last June PSW posted an "upcoming road test" article about Raimonds Skuruls's TDA software, and how it was being tested in the US. Has anything come from that? They never posted the actual road test, so I was wondering if anyone else has been using it. APL Audio now has a half rack hardware box to apply the TDA settings with Analog and AES/EBU i/o. Has anyone tried it?

Last June PSW posted an "upcoming road test" article about Raimonds Skuruls's TDA software, and how it was being tested in the US. Has anything come from that? They never posted the actual road test, so I was wondering if anyone else has been using it. APL Audio now has a half rack hardware box to apply the TDA settings with Analog and AES/EBU i/o. Has anyone tried it?

-Lee

I have the software and will produce the review next week for the magazine. Stand by...

Last June PSW posted an "upcoming road test" article about Raimonds Skuruls's TDA software, and how it was being tested in the US. Has anything come from that? They never posted the actual road test, so I was wondering if anyone else has been using it. APL Audio now has a half rack hardware box to apply the TDA settings with Analog and AES/EBU i/o. Has anyone tried it?

-Lee

I have the TDA software and the ALP1 … they’re great.

I have used TDA to help me EQ a system for musical production - probably the best results I have ever had.

I also used it in designing this project – https://soundforums.net/threads/11317-New-DIY-Mid-High/page16 see post 74

One year or so later, is this still a hot topic? I'm a DIYer and have been using TDA for about 6 months and I like to write so let me jump in.

TDA, from Acoustics Power Labs, provides a new way of looking at the responses of speakers in rooms. Its Intuitive 3D Display of sound level vs arrival time and frequency spotlights speaker and room anomalies.

An impulse response shows energy arrival over time; TDA shows both the amplitude and the spectrum of that arriving energy over time. It is useful in both the design and fine tuning of the speakers themselves, especially their crossovers, and in integrating those speakers into rooms. TDA is the analysis step on a ladder of tools from APL that provide increasingly more powerful equalization solutions for adapting speakers to listening spaces.TDA uses a sine sweep to obtain an impulse response that can be viewed by use of the IR and Log IR buttons on its control panel. The calculated impulse response contains non-linear distortion information that can also be displayed. Distortion can also be seen as a series of spikes before the main peak of the IR in the log IR view. Third party tool IRs can also be imported and viewed.

The obtained IR is then processed by TDA at 126 frequencies, 12 log spaced points per octave to create its unique 3D display - a 3-dimensional map of normalized sound pressure plotted with delay and frequency along the X and Y axes and SPL encoded in both color and on the Z axis. Boundary effects, room modes, timing alignment issues with multi-way speakers, and frequency response aberrations are easily recognized in this presentation. This 3D map is a very good tool for quickly evaluating a new speaker in a familiar space or a familiar speaker in a new room or position.

TDA also provides conventional frequency, delay, and phase response graphs. TDA attempts to separate the direct response from reflections based on time of arrival and succeeds in doing so down into the modal zone to an extent limited by room modes, near reflections and the increased difficulty of precisely determining the time of arrival of low frequencies. This time selectivity enables us to see both the direct response, otherwise viewable only as an anechoic or quasi-anechoic measurement, and the effect of the room. It’s not a coincidence that the human ear has the same/similar ability to separate direct sound from reflections and the same/similar limitations.

TDA’s frequency response graph, the AFR, shows this separated direct response while its 3D display allows you to judge the extent to which it stands above room and boundary effects. Where it is reflection free, it is minimum phase and thus can guide equalization; other products from APL use multiple AFRs taken at multiple positions to do just that.

The review will continue in additional posts. Meanwhile, you can follow this link to the APL websitehttp://aplaudio.com/conc2/products/tda/ (http://aplaudio.com/conc2/products/tda/)

The TDA 3D display has seductive appeal. Its just nice to look at, especially when showing a measurement of a good speaker.

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An ideal speaker would have a response as shown in the attached figure, the looped back response of a soundcard. A straight line along the frequency response axis representing the direct sound would indicate perfect time alignment of a multi-way speaker. The sharpness of this blade would indicate the absence of diffraction and near-in reflections. Delayed reflections would show up as lighter colors to the right of the blade in the area now dark blue. These are likely to be present in a real room measurement but not in a soundcard loopback.

In TDA, one can choose which time values to display. I’ve offset the display above by 10 ms. and limited the display range to a 30 ms. range. Unless offset, time values default to being relative to the arrival of what would be the impulse peak in a conventional IR. One has the option of connecting a reference loopback via a 2nd soundcard channel if there is a need to know absolute time values – perhaps to compare sound channel latency for various configurations or FIR filters.

The Z-axis defaults to a normalized scale of 0-1 as shown here but can be set into a high dynamic range mode with a logarithmic scale and a configurable maximum value. The normalization sets the amplitude of the direct response line in the 3D display at each frequency to 1. The un-normalized direct response values appear in the AFR graph. Just past the 25 ms. mark in this loopback response, you can see a ghostly white line parallel to the frequency axis. This was determined to be sound card internal crosstalk between its microphone preamp and line level output due to its monitor mixer circuit. I missed this ghost evaluating the sound card using conventional FR and IR measurements; had I noticed it then I would likely have a different sound card. With TDA you see things easily overlooked or misunderstood in conventional speaker measurement

A close in measurement is used when the goal is to evaluate a speaker or its crossover. Some room effects are visible in the 1m measurement shown below but the speaker does dominate. This is the response of a 3 way, full range speaker with near perfect time alignment and some crossover phase shift evident. The speaker is nestled tightly into a room corner, eliminating front and sidewall reflections but leaving it susceptible to floor and ceiling reflections that are indeed visible in the display. Some floor absorption was used near the microphone to limit the disturbance to the midrange response from the floor bounce. Except for this and some relatively thin carpet, the room is untreated. Delayed reflections from the back wall of the room are visible near the 38 ms. mark.

Because of the dominance of the speaker over the room modes, TDA correctly identifies the direct response well down into the bass, although the direct response line does widen at lower frequencies.

It’s easier to determine the frequency ranges affected by room modes and reflections by looking down on the 3D map from above. The TDA “pl” display option is used in the attached figure.

It’s important to be able to distinguish between the properties of the speaker and properties of the room in these displays. To do that we must first understand the speakers; in particular, that they are corner speakers and rely on supportive reflections from nearby floor and walls to support the bass and to a lesser extent the lower midrange.

The wedge of red level response embracing the black vertical line of the direct response from 50 Hz up to 1000 Hz is the result of reflections from the floor at a nominal delay of 2.8 ms and from the ceiling as well where the delay is greater than 6.9 ms. For the corner woofer, this is floor support and does indeed seem to be propping up the direct response line in the 3D display. The crossover between the woofer and the multiple entry horn that sits on top of it is at 350 Hz and shows in the PL graph as an abrupt narrowing of the boundary support wedge. This narrowing continues gradually as frequency rises due to increasing directivity of the speaker and the increasing absorption from the carpet and temporary floor absorber used for this measurement. Unfortunately, what was boundary support for the woofer becomes boundary interference for the midrange resulting in nulls in the 400 -500 Hz range in unsmoothed measurements taken at the listening position.

Above the bass, we are seeing the speaker and not the room; that is abundantly clear in the 3D display. The color shading close in to the direct response line at higher frequencies is due to speaker imperfections and can be improved by minimum phase equalization. Strong room resonances appear as the red streaks at 50 and 70 Hz and a weaker one at 180 Hz. In a steady state, the standing waves of a room mode would develop but in a transient sine sweep or impulse response they can only be called resonances. The vertical white streak at 38 ms. has a timing consistent with a reflection from the back wall of the room.

TDA can also show frequency response curves. While the 3D and pl displays are normalized, the AFR is the un-normalized frequency response recorded along the black direct response line of the 3D display. Room resonances, or their effects, visible in the 3D display can also be seen in the AFR. To wit, the room resonances at 50 and 70 Hz result in a peak at 50 Hz followed by a shallow null at 70 Hz.

To the extent that the processing succeeds in separating the direct response from reflections, the effect of reflections won’t show up in the AFR. In this measurement, supportive floor reflections have elevated the bass and lower midrange but no reflection nulls are visible.

Before we go any further, let me tell you about the speakers whose measurements are being used in this review.

These 3-way corner speakers use active, DSP crossovers. They each consist of a multiple entry conical horn containing a compression driver tweeter and four 4” midranges sitting on top of a 15” sealed, slot loaded woofer nestled tightly into each of the room’s two front corners. Time alignment was achieved by setting DSP delays to align the IR peaks of individual drivers with their crossover filters and PEQs enabled. Time alignment was then confirmed with TDA. The delay bend just past the mid to CD crossover at 950 Hz is due to the mid-tweeter crossover; it increases with increasing crossover filter slope. A room resonance or reflection also affects the response there. The only hint in the 3D display of the woofer-mid crossover at 350 Hz is the narrowing of the floor support wedge there, indicating excellent time alignment. Both XOs use only 12 dB slope electrical filters, achieving 24 dB acoustical slopes. Neither crossover shows a phase wrap in its conventional frequency/phase response graph.

The next step is to look at the speaker's response out at the listening position.

When the mic is moved out to the primary LP, 4m from the corner speakers, the room dominates the measurement, especially in the lower registers. The TDA display isn’t as pretty a picture there but then it’s an untreated room. The low frequency red and black horizontal streaks are room resonances. Vertical streaks and points or blobs are reflections. Despite the resonances and reflections, the direct response stands out down into the modal region, which is encouraging.

To diagnose the room modes and reflections evident above, we again look at the TDA PL display and at the AFR graph to see their effect on the frequency response. The pl display below is configured to high dynamic range mode with a 20dB logarithmic scale and annotated to correlate the 3D display with the corresponding AFR.

In this room, modal effects dominate below 200 Hz and a general lack of bass damping is evident. Without TDA, seeing nulls in the 100-200 Hz region in conventional frequency response measurements and misled by SBIR and room mode calculators that assumed a rectangular room, I initially believed those frequency response nulls were due to ceiling reflections. With TDA, I at first thought it was a more distant reflection. Gradually, I came to understand that it was a longitudinal room mode and that the listening position just happened to be sitting in a null of the mode’s standing wave. Moving the microphone just 2’ forward and out of the null gave a cleaner measurement, confirming the diagnosis.

Several reflections and resonances in Figure 6 were annotated and traced back to their sources by iteratively moving absorber panels and re-measuring. Marker 1 shows the impact of the longitudinal room mode just discussed. In general, the bass is elevated in part due to the modes and floor support but also as part of a house curve. That voicing needs to be redone based on a set of measurements taken over the listening window. Marker 2 points at reflections from objects on the front wall between the speakers, the flat panel TV and audio equipment rack. Marker 3 points at reflections from the unterminated conical horn mouth itself. Marker 4 shows multiple reflections or modes between 400 and 500 Hz, where floor bounce nulls occur.

Eventually, I placed bass traps in the corners above the speakers, which reduced the room modes dramatically. Using the AFR as a guide, I attenuated the modal peaks at 80 Hz and 180 Hz and obtained the vastly improved measurement below in which the direct response stands out well down into the bass.

Prior to TDA, I found even windowed measurement data taken at the LP in room overwhelming – too many frequency response nulls and impulse response peaks to make sense of easily. TDA helped me distinguish between room modes and reflections, to locate the sources of reflections, and immediately showed me the effectiveness of treatments I applied. With the speaker’s direct response now prominent in the measurements taken at the listening positions, I’m confident the room equalization process will result in a more than satisfactory listening experience.

The discussion so far has been focused on the unique2D and 3D maps of APL_TDA. Let’s look at its controls and see how else it can present measurement data.

In the upper left hand corner of the main screen, TDA has shown me all the audio devices it has found on my system. Below it, I tell TDA which ones to use for input and output. Then I just click in the green “RUN MEASUREMENT” box and, after a delay for computation, get a measurement and a result display. Below the run button is a column of 4 grey buttons that allow the saving and subsequent importing of recorded measurement data and the impulse response computed from it. TDA automatically saves measurement impulse responses in a “HISTORY” directory below its executable.

Results appear in the large window to the right. Display options, as shown in the column of buttons immediately to the left of the main display window are: recorded measurement data, IR normalized, IR Log, the 3D and pl displays we’ve seen, AFR – the frequency response, DFR – delay vs frequency and non-linear distortion (NLDA). If the “SW” box is checked then pressing any of these buttons shows the curve in a new window from which the graph can be saved to memory in a variety of formats.

Impulse Response DisplayThe figure in the previous posted shows the IR Log display. The height of the peak above the noise floor shows the SNR ratio of the measurement. Reverberation time can be determined from the slope of the response envelope following the main IR peak. Zoom in/out controls are available. A linear IR graph is also available.

Visualization of Reverberation TimeAnother way to visualize reverberation decay is with the PL display zoomed out to 300 ms. and configured for 60 dB dynamic range as in the picture below.

Frequency, group delay, and phase delay response curves can be viewed with varied FFT-Q window settings to remove reflection interference from the frequency, phase, and group delay response graphs. The controls for FFT-Q window analysis are below the lower left corner of the main display screen. A low Q setting represents a narrow time window, better able to keep out reflections. A high Q keeps the window open longer making more detail of the measured response visible. The “FFT-Q” is roughly equivalent in effect to a frequency dependent window in other measurement tools.

With Q set to 8, the widest the window can open, we get a detailed picture of the AFR of a 1m measurement in the first attached figure.

Reducing Q to 1.8, the narrowest opening, results in this AFR from the same measurement in the second attached figure.

With Q set to 8, the curve in the first attached figure is affected by the room to a high degree, despite the close in measurement, no doubt because of the speaker's use of boundary support in the bass and low midrange.

Narrowing the window by reducing Q to 1.8 in the second attached figure, we see a curve more representative of the group delay of the woofer and its cross over and less of the room.

The subtract minimum phase option has a further effect on the FFT-Q windowed response. Viewing GDR or PDR with a narrow window and minimum phase subtracted in the third attached figure reveals the residual non-minimum behavior of the response, most often due to the crossover and/or equalizer.

The rising group delay in the bass in the third figure is in good agreement with the woofer simulation shown below in the fourth attachment over at least the 20 to 100 Hz range. The small bump just past 1 KHz is due to the mid-CD crossover and might be eliminated with further development of the crossover. Using TDA’s GDR and PDR curves with minimum phase subtracted allows one to zero in on crossover phase and group delay using indoor measurements for the final stage of crossover tuning.

APL_TDA has become my favorite tool for evaluating my DIY speaker efforts in room, the listening position itself, and the effectiveness of various room treatments I’ve been experimenting with. It has helped me see issues that I missed or misinterpreted with conventional tools. I still use those programs for simulation and crossover development but TDA is where I turn to see (as opposed to hear) how well I’ve done in speaker and system design and implementation and integrating the speaker into my room.

Well, that was the end of my review but I want to continue to probe the limits of TDA's ability to discriminate between the direct response and reflections. That is what its utility depends on and we need to understand its limits.

To explore this topic, we will look at a speaker both alone and in the presence of a strong reflection, delayed by 5 ms. and down only 3 db. For most small rooms, a 5 ms. delayed reflection would have traveled more than twice as far as the direct sound and would be down by more than 6 db. Thus, it is a worse than worst case situation and has been created artificially rather than by measurement.

The speaker has 4 ways with crossovers at 200 Hz, 1.6 kHz, and 5 kHz that use 4th order LR filters. Measured close up and free from room effects, TDA shows a clean pl graph, with group delay increasing towards low frequencies and an impressively flat frequency response.

The AFR and GDR below were taken with FFT-Q = 8. The GDR below is plotted with minimum phase subtracted, leaving the crossover group delay. The slight group delay ripple at 50 Hz correlates with the European AC mains power and is an equipment artifact, not part of the speaker’s response.

A couple more graphs of the reflection free response before I show what happens when the strong borderline early reflection is added.

The first attachment is the GDR obtained by subtracting minimum phase. This is the crossover group delay. If the tool can show us the same group delay in the presence of a strong reflection, we should be impressed.

The second attachment is the PFR or phase frequency response. We can see the crossovers in this phase response.

Next, we will look at the same waveforms in the presence of a reflection delayed by 5 ms. and down only 3 dB. The TDA 3D display shows twin blades at high frequencies, with the direct response blade bending towards the reflection as the mid-bass is approached.

The effect of the reflection is seen more clearly in the pl graph in the 2nd attachment below.

TDA separates out the reflection with no difficulty evident above 1.5 KHz but with increasing difficulty as frequency lowers. The direct response line broadens as frequency lowers and ripples of increasing magnitude appear in the direct response.

The reflection cause similar distortion in the phase response, shown in the first attachment.

However when we subtract minimum phase the undistorted GDR and phase responses appear, second and third attachments.

The GDR and PFR obtained by subtracting minimum phase in the presence of the strong reflection match those obtained in the absence of a reflection. Thus, with TDA, one can evaluate crossover timing using indoor measurements in a reverberant environment.

The new TDA IM intermodulation distortion measurement software is just added to TDA software family.http://aplaudio.com/conc2/products/tda-im/Now we can evaluate the IM distortion`s dependence of frequency in form of frequency responses.